The technological field relates to assembly, test and packaging processes, structures and materials in semiconductor device manufacturing, and qualification, reliability and failure analysis processes.
Lead frames or other supports for semiconductor devices provide a stable support pad called a die pad for firmly positioning a semiconductor integrated circuit (IC) chip or die within a package or on another support for the chip. In addition to the chip die pad, a lead frame offers a plurality of conductive segments to bring various electrical conductors into close proximity to the chip. Types of package materials for an IC chip may be ceramic or plastic, for instance. The package encloses and protects the IC chip, and the exterior of the package is labeled, as may be visible on a printed circuit board in a consumer device or other electronic unit.
Single-piece lead frames are suitably manufactured from thin sheets of starting metal. The desired shape of the lead frame is stamped or etched from an original sheet. The conductive lead frame segments bring various electrical conductors into close proximity of the chip. The end of the lead frame segments nearer to the IC chip (“inner” ends) are electrically and mechanically connected to so-called connections or connectors, e.g., wires, that go to contact pads on the IC surface. The IC contact pads are also called bond pads and are fabricated on, and connecting internally to, the microscopic integrated circuit structures deposited or made in, on and over a semiconductor substrate. The spatial distances or gaps between the inner ends of the lead frame segments and the IC contact pads are thus bridged by the connections, typically thin metal wires such as gold individually bonded to the IC contact pads and to the lead frame segments. Consequently, the surfaces of the provided inner ends of lead frame segments are chosen to be metallurgically suitable for stitch-attaching the connectors. The end of the lead frame segments remote from the IC chip (“outer” ends) are electrically and mechanically connected subsequently to external circuitry such as a printed circuit board by soldering.
The lead frame provides a stable frame and die pad for a chip. A die attach material is used with dispensed die attach adhesives to form an adhesive bond between the IC die or and lead frame. A ceramic package may be used, or a low-cost molding instead may be provided to plastic-encapsulate the IC die and connecting wires. BGA (ball grid array), QFN (Quad Flat No-lead) and SON (Small Outline No-lead) are among many forms of such packaging known today. The packaged chips are borne by the lead frame and are cut apart from one another and the cut-off exterior supporting portion of the lead frame is discarded.
Among its other functions, a lead frame can provide a path for thermal conduction of heat to leave the IC chip. Problematically, however, the dispensed die attach adhesive generally cures into a resin layer of adhesive material that interposes a thermal barrier skin effect between the die and lead frame even as the die and lead frame form an adhesive physical bond or die attach interface between them. The thickness of the die attach adhesive is called the bond line thickness BLT.
The adhesive physical bond is subject to problems including physical integrity and moisture entry which can limit IC life and introduce failures. Stress-induced die attach delamination (separation or tearing) is a leading package-related failure mechanism for Moisture Sensitivity Level (MSL) failures. Even beyond the thermal barrier skin effect noted above for a die attach that has not failed, stress-induced die attach delamination failure further degrades the thermal path as well.
Various approaches have been proposed by others to solve these problems. Carbon nanotubes (CNTs) have high thermal conductivity and other remarkable properties, and have been discussed for possible chip application, but discovering practical ways of providing useful CNT structures and applying CNTs for actual volume manufacturability challenges the industry. Inventive improvements for solving the above problems and other problems continue to be of high interest and most desirable in this art because of the manifest importance of volume manufacturability, reliability, performance and economy to the semiconductor industry and users alike.